1. Technical Field
The present invention generally relates to a mandrel for winding a coil and methods for its use. More particularly, the present invention relates to an axially adjustable mandrel for winding a coil and methods for its use.
2. Background Art and Technical Problems
Precision wound coils are used in various applications, such as in fiber optic gyroscopes. Coils for such applications must be wound in a precise and symmetric manner. Any non-symmetry will introduce errors into the gyroscope. A mandrel is used to precisely wind such coils. A mandrel is a form upon which a coil can be wound and subsequently processed. The coil may include a fiber optic coil, an electrical coil, or the like. For purposes of illustration only, without limitation, the coil will hereinafter be described as a fiber optic coil. The processing may include varying the temperature of the coil, subjecting the coil to chemical treatment, or the like, A coil material such as a fiber optic coil is wound on the mandrel in a precise manner to optimize coil symmetry. During the winding process, the mandrel serves to aid in the precise alignment of the coil. An adhesive material may be applied to the fiber prior to or during the winding process. After the winding process is completed, the coil is heated, cooled, and/or left at room temperature to cure the adhesive. During the subsequent curing, the mandrel serves to maintain the alignment of the coil. Following the curing process, the coil is removed from the mandrel to leave a stand-alone coil.
In the past, mandrels have consisted of a core about which the coil is wound and two fixed flanges at the ends of the core to define the edges of the coil. The winding of the first layer of the coil is critical because the first layer becomes the form and pattern which determines the spacing and positioning of subsequent layers. Ideally, the axial spacing between the end flanges is selected to aid in the alignment of the first layer. If the axial spacing is too large or too small, for example, because of variation in the fiber diameter of the coil material, the coil winds will not be tightly spaced and the necessary precision of the first layer cannot be achieved. To overcome this problem, some mandrels have used a grooved core to correctly position the first layer of the wound coil. However, a different grooved core must be used for each coil having a different fiber diameter which is impractical and costly. In addition, fiber diameters have tolerances which may change when the coil is processed. Consequently, a grooved core having a predetermined number of turns per layer cannot accommodate such changes in fiber diameter tolerances.
Other prior art mandrels have a smooth surface on the mandrel. Winding a coil onto a smooth surfaced mandrel is difficult because there are no grooves to guide each turn of the coil onto the mandrel surface. The prior art mandrels, both grooved and smooth surfaced, have used fixed flanges with a predetermined coil height where the coil height is the axial distance between the two flanges. The coil height of fixed flange mandrels cannot be adjusted during winding or curing. The fixed coil height may cause one or both of the fixed flanges to misshape or crush the coil during curing. Such a problem depends on the amount of expansion and/or shrinkage of the coil during curing. The mandrels of the prior art having fixed flanges do not allow for expansion and/or contraction of the coil material during a curing process. Accordingly, expansion of the coil material against the fixed flanges may cause unacceptable distortion in a precision coil, and contraction may misalign the tightly spaced layers of coil.
In addition, the effect of a temperature change on a grooved mandrel during curing is even more dramatic. During such a curing process, a coil that is wound into the grooves may expand at a different rate than the mandrel material. This differential expansion between the coil and the grooved mandrel causes a ripping effect of the base layer of the coil. In essence, this affects the final symmetry of the coil and can cause permanent stress points in the coil which are highly undesirable.
Due to the need for precision wound coils and in view of the problems associated with prior art mandrels and methods, a need exists for a mandrel having an adjustable flange that can move axially during coil winding and/or processing.
In accordance with one embodiment of the invention, a mandrel is provided that aids in the precise winding of the first and subsequent layers of a coil. The mandrel also adjusts to accommodate the thermal expansion and/or contraction properties of the coil material during a curing operation. In accordance with one embodiment of the invention, a mandrel includes a smooth core combined with one stationary flange and one adjustable flange. The first coil layer is wound onto the core starting at the stationary flange end. The adjustable flange is then brought into close contact with the last winding at the opposite end of the first layer of the coil and is fixed in this position. Subsequent layers of the coil are wound over the first layer, using the two flanges as edge guides. When the coil winding is completed, a spring mechanism is used to hold the adjustable flange against the edge of the wound coil with a predetermined force such that the mandrel is axially loaded and the coil height may vary during a subsequent curing process. During the curing process, the temperature of the coil and the adhesive associated with the coil may be varied. The predetermined force is selected to allow a controlled axial movement of the adjustable flange as the coil material undergoes changes as a result of the temperature change. The controlled movement accommodates expansion and/or shrinkage of the coil material, but maintains the windings in proper alignment and position.